1. Field of the Invention
This invention relates generally to broadband radio-frequency impedance transformers. More particularly, the invention relates to broadband transmission line transformers with non-integer turns ratio (fractional ratio transformers) and mostly for high power application.
2. General Description of the Prior Art
A particular class of RF impedance transformers with maximum achievable bandwidth and low insertion losses is a class of transmission line transformers that plays an important role in various RF systems, from low power up to high power levels.
The main frequency limitation factors of these transformers are shunt inductance at lower frequencies and electrical length of transmission lines at higher frequencies. These two contradictory factors determine the achievable bandwidth of transformers. Impedance transformers with diverse circuit models, having different interconnections of transmission lines and impedance transformation ratios, have different limitations influenced by these two factors. As result, greater or lower bandwidth can be achieved.
Widely used impedance transformation ratios are 4:1, 9:1 and 16:1 (integer turns ratios), and 1.5:1, 2:1, 3:1 (fractional turn's ratios). The latter ones are more difficult to realize at wide bandwidths, especially for high power application. Various circuit diagrams of transmission line impedance transformers are presented in book of Jerry Sevick “Transmission Line Transformers.” Among the impedance transformers with non-integer turns ratios, the most necessary is 2:1 impedance transformer. A typical structure includes a two-way power combiner/divider, which consists of a combiner/divider itself and a 2:1 impedance transformer.
All of these RF transformers have multi-octave bandwidth and use generally ferrite toroids or other ferrite configurations. Due to high magnetic permeability of ferrite transformers, shunt inductance is high enough and it is possible to realize multi-octave bandwidth with admissible electrical length of transmission lines.
In high power transformers (5-100 kW), magnetic flux in ferrite is also high and introduces harmonics and intermodulation products.
Furthermore, for these transformers, hysteresis losses (heat dissipation) limiting power handling capability may require a liquid cooling system. Such transformers are heavy, expensive and can not be used in certain environmental conditions.
Many attempts to develop high power, broadband transformers without ferrite have been made. In this case the high-pass correction usually used for partly compensation of relatively small shunt inductance. In simplest case it may be one series connected capacitor at the input or at the output of transformer.
In spite of this, for achieving multi-octave bandwidth, especially at high power, the electrical length of the transformer's transmission lines should be great and high frequency limitation occurs. Additional low-pass correction compensates this effect to some extent. All of these corrections make transformers more complicated and expensive.
In addition, for transformers with fractional turn ratios, the impedance ratios in some practical cases are not close enough to integer numbers and, consequently, even if the transformer is ideal some mismatch occurs. For example, when typical turns ratio is 3/2, the corresponding impedance's ratio is 2.25, and with respect to required impedance transformation ratio equal to the calculated VSWR=1.125. Practically, in this case value of VSWR will be higher.
Furthermore, in combining the power of several amplifiers, a two—or more stage combining system is usually used. If each stage inserts some particular VSWR, the overall VSWR in the worst case is a product of its individual values. To decrease the above-mentioned theoretical value, the turns ratio 7/5 instead of 3/2 may be used, for example. A corresponding transformer is too complicated, especially for high power application. Besides, admissible electrical length of its transmission lines should be relatively small and the highest operating frequency decreases.
The factors discussed above are applicable to impedance transformers that are unbalanced, balanced and baluns (balanced-to-unbalanced). Frequently it is difficult to provide good balance for high power, broadband baluns, especially for fractional turns ratios.
In a prior art balun with 2.25:1 impedance ratio (U.S. Pat. No. 5,767,754), an additional transformer winding is used to improve balance. This winding introduces capacitive shunt effect that increases mismatch. Besides, balance can not be perfect due principally to the asymmetry of circuit models and the influence of stray elements, especially for high power applications. Different longitudinal voltages on the windings also introduce additional difficulties at high power levels.
Another approach is a chain connection of two transformers in different combinations (see books of Jerry Sevick: “Transmission Line Transformers” and “Building and Using Baluns and Ununs,” CQ Communications Inc., 1994). This approach is too complicate at high power levels and the balance is not good enough due to stray elements in real design.
In view of the above, it is an object of the present invention to provide a more effective, high power broadband impedance transformer.
It is another object of the present invention to provide a high frequency, high power transformer with unbalanced ports that is simple in construction and has a wide bandwidth without ferrite.
It is still a further object of the present invention to provide an unbalanced impedance transformer without ferrite having a multi-octave bandwidth ratio up to 20:1.
Still another object of the present invention is to provide a high power, broadband unbalanced transformer with a fractional turns ratio, and specifically to provide a 2:1 impedance transformation ratio.
Yet another object of the present invention is to provide a broadband, unbalanced transformer with a simple correction.
It is still a future object of the present invention to provide a broadband, unbalanced impedance transformer having very small mismatch with respect to standard nominal port impedances.
It is another object of the present invention to provide a high frequency, high power transformer with balanced ports that is simple in construction and has wide bandwidth without ferrite.
It is still a future object of the present invention to provide a broadband balanced-to-unbalanced impedance transformer (balun) having all above mentioned properties and good balance in entire frequency band.